[clang-tidy] Improve `ExceptionSpecAnalyzer`s handling of conditional noexcept expressions (#68359)

The previous code was pretty messy and treated value dependant
expressions which could not be evaluated the same as if they evaluted to
`false`. Which was obviously not correct.

We now check if we can evaluate the dependant expressions and if not we
truthfully return that we don't know if the function is declared as
`noexcept` or not.

This fixes #68101